You protagonists somehow happen upon a cabin in a forest regardless of the origin location, NYC Central Park, Serra Nevada Mountains, Great Smoky Mountains, or even in a European Forest. At first seeking shelter from the elements (rain or night) but one can never leave as walking away in any direction leads back to the cabin. The victims utter a name an animal when approaching the cabin for the first time, be it name calling, referencing, or simple manors, that person is cursed to slowly become hermaphroditic versions of that animal (normally mammals, but you can be creative) in lustful sexual waves. A body part changes with each arousal or pleasure, sometimes both, becoming permanent when that person cums. It does not matter how pleasure is induced masturbation, sex, or rape. However if the victim(s) managed to resist cuming then the changes will revert, often giving them previews of the next change too come (can be different thou). The first change is always the tail, but early changes afterwards are small, ears, noses, and sexual organs (including gaining the other gender), but propagate hereon into bigger and bigger changes, arms, hands, feet and legs, head, and various parts of the torso, both internal and external. Part of the fiction is the victims coping and living with the changes, trying to adapt and make the most of their altered bodies and at all costs avoid sexual encounters. Time skips and adding more victims later on are allowed. The victim(s) will subconsciously act more like the animal they are becoming and take on their eating habits, but still retain self control up to a point near the end. At which point the animal mind becomes more dominate, eventually gaining full control with only a short reclamation if given their human name. Upon becoming a full animal they will leave into the surrounding woods, sexually preying upon new victims if they are found aimlessly wondering the woods.
The cabin itself will always have electricity, remain stocked of food and supplies including some basic clothing, repair itself from any damages to it, nearby trees, or its furniture, and it’s completely isolated from the outside world. It will also modify itself and the surrounding area to either accommodate certain conditions, farther tempt its victims or ensnare the victim(s) in a location such as: arriving with a pack donkey or on horseback and a small pen appears nearby, arriving with a dog and a doghouse appears near the cabin, on four hooves getting trapped in a coral with a ‘friend’, or a pond appears for fishing, idle hands, or upon becoming an otter. (Note: former victims will also be drawn to these sites as well, complicating matters.) All of these changes occur while none of the current occupants are looking at that location and the animals just don’t care when they do see the alterations.
Animals brought to the cabin will have waves of lust toward their human counter parts (not each other thou) and typically complete their own changes in just one (opposing gender parts) or two goes if ‘fixed’ (either regaining lost or opposing parts during arousal). After becoming complete sooner or later, any accommodations for them will mysteriously disappear, releasing them into the woods.
No one knows if there is a way out, how to how to pull it off, or revert any and/or all of the changes. Or for that matter how this dammed curse got started in the first place. But people will survive and continue living even if it means surrendering to wild side of life.